SU-8 Layer Research Articles

PDMS-PDMS Micro Channels Filled with Phase-Change Material for Chip Cooling.

This paper reports on a chip cooling solution using polydimethylsiloxane (PDMS) based microfluidic devices filled with n-Octadecane. A thick SU-8 layer of 150 µm is used as the master mold for patterning PDMS fabrication. With the SU-8 mold, patterns with straight lines at microscale have been fabricated with standard micro-electro-mechanical system (MEMS) technology. Thermal polymer bonding technique is used to bond the PDMS pattern directly to a flat polydimethylsiloxane (PDMS) film which results in the sealed microchannels. n-Octadecane as a phase-change material has been successfully filled in the microchannels using a dispensing machine. Infrared thermal image shows a sharp contrast of the temperature distribution between the chip with n-Octadecane and the empty chip during the same heating process. This result indicates an efficient cooling performance of the microchannel device with phase-change material. A thermal stimulation test demonstrates that a 16 °C-lower temperature difference can be achieved. This microchannel device, benefited from the flexibility of PDMS substrate, shows specific advantages in meeting the need for the heat dissipation of flexible electronics such as flexible displays, electronic skins, and wearable electronics. Latent heat of the phase-change material can keep the temperature of devices relatively lower over a period of time, which shows potential application values on discontinuously active flexible electronic devices.

Design and measure of a tunable double-band metamaterial absorber in the THz spectrum

We demonstrate and measure a hybrid double-band tunable metamaterial absorber in the terahertz region. The measured metamaterial absorber contains of a hybrid dielectric layer structure: a SU-8 layer and a VO2 layer. Near perfect double-band absorption performances are achieved by optimizing the SU-8 layer thickness at room temperature 25 °C. Measured results show that the phase transition can be observed when the measured temperature reaches 68 °C. Further measured results indicate that the resonance frequency and absorption amplitude of the proposed metamaterial absorber are tunable through increasing the measured temperature, while structural parameters unchanged. The proposed hybrid metamaterial absorber shows many advantages, such as frequency agility, absorption amplitude tunable, and simple fabrication.

Design and measuring of a tunable hybrid metamaterial absorber for terahertz frequencies

A tunable hybrid metamaterial absorber is designed and experimentally produced in THz band. The hybrid metamaterial absorber contains two dielectric layers: SU-8 and VO2 layers. An absorption peak reaching to 83.5% is achieved at 1.04 THz. The hybrid metamaterial absorber exhibits high absorption when the incident angle reaches to 45°. Measured results indicate that the absorption amplitude and peak frequency of the hybrid metamaterial absorber is tunable in experiments. It is due to the insulator-to-metal phase transition is achieved when the measured temperature reaches to 68 °C. Moreover, the hybrid metamaterial absorber reveals high figure of merit (FOM) value when the measured temperature reaches to 68 °C.

Ultra-repellency of Al surfaces: design and evaluation

Aluminum (Al) surfaces with ultra-repellency as well as desirable robustness were designed and fabricated. With photolithographic patterning of a thick SU-8 layer and sputtering of a thin Al film, re-entrant micro-pillar textured Al surfaces were prepared. After derivatization with perfluoroalkyl phosphoric acid (FPA), the textured Al surfaces showed ultra-repellency for a wide variety of liquids. The contact angles (CAs) of deionized (DI) water, hexadecane and dodecane were larger than 150°, and those of methanol and ethanol were larger than 100°. The sliding angles (SAs) of DI water, hexadecane and dodecane were 5°, 10°, and 10°, respectively, showing excellent superamphiphobicity. The SAs of methanol and ethanol were in the range of 20°–30°. The robustness of the ultra-repellent Al surface was evaluated by three parameters: robust height (H*), robust angle (T*) and robust factor (A*). For the DI water probing, the values of the parameters are H* ≈ 403, T* ≈ 119 and A* ≈ 92, respectively, indicative of a desirable robustness. We clarified that only re-entrant structures can support composite liquid–solid–vapor interfaces when the corresponding Young’s CAs are smaller than 90°, and the function of the nanometer structures of the hierarchical textures which were widely adopted to fabricate superamphiphobic surfaces is to help construct re-entrant structures. FPA derivatization is effective in lowering the surface energy of Al surfaces, combining with re-entrant textures to provide a simple and high throughput approach to ultra-repellency for a wide variety of liquids.

Designer Neural Networks with Embedded Semiconductor Microtube Arrays.

Here we present a designer's approach to building cellular neuronal networks based on a biocompatible negative photoresist with embedded coaxial feedthroughs made of semiconductor microtubes. The diameter of the microtubes is tailored and adjusted to the diameter of cerebellum axons having a diameter of 2-3 μm. The microtubes as well as the SU-8 layer serve as a topographical cue to the axons. Apart from the topographical guidance, we also employ chemical guidance cues enhancing neuron growth at designed spots. Therefore, the amino acid poly-l-lysine is printed in droplets of pl volume in the front of the tube entrances. Our artificial neuronal network has an extremely high yield of 85% of the somas settled at the desired locations. We complete this by basic patch-clamp measurements on single cells within the neuronal network.

WR-3 Waveguide Bandpass Filters Fabricated Using High Precision CNC Machining and SU-8 Photoresist Technology

This paper presents two WR-3 band (220–325 GHz) filters, one fabricated in metal using high precision computer numerically controlled milling and the other made with metallized SU-8 photoresist technology. Both are based on three coupled resonators, and are designed for a 287.3–295.9-GHz passband, and a 30-dB rejection between 317.7 and 325.9 GHz. The first filter is an extracted pole filter coupled by irises, and is precision milled using the split-block approach. The second filter is composed of three silver-coated SU-8 layers, each 432 μm thick. The filter structures are specially chosen to take advantage of the fabrication processes. When fabrication tolerances are accounted for, very good agreement between measurements and simulations are obtained, with median passband insertion losses of 0.41 and 0.45 dB for the metal and SU-8 devices, respectively. These two filters are potential replacements of frequency selective surface filters used in heterodyne radiometers for unwanted sideband rejection.

Study of THz-Plasmon hybridization of a loop Yagi-Uda absorber

In this article we present a three-dimensional loop Yagi-Uda array for efficient, polarization independent and directional absorption of THz radiation over a narrow frequency range (f0 = 0.657 THz & Q factor = 7.5). Unit cell of the array consists of three vertically stacked gold micro rings separated from each other by 30 µm thick SU-8 layers. The proposed array also exhibits a filtering response in its transmittance spectrum. The characteristics are explained by plasmon hybridization method. The transmission, reflection and absorption spectra of the structure are measured and they show a good agreement with corresponding simulated results.

Fast electrochemical membrane actuator: Design, fabrication and preliminary testing

An actuator based on water electrolysis with a fast change of voltage polarity is presented. It demonstrates a new actuation principle allowing significant increase the operation frequency of the device due to fast termination of the produced gas. The actuator consists of a working chamber with metallic electrodes and supplying channels filled with an electrolyte. The chamber is formed in a layer of SU-8 and covered by a flexible polydimethylsiloxane membrane, which deforms as the pressure in the chamber increases. Design, fabrication procedure, and first tests of the actuator are described.

Implementation of Wafer Level Packaging KOZ using SU-8 as Dielectric for the Merging of WL Fan Out to Microfluidic and Biomedical Applications

Abstract Due to its versatility for high density, heterogeneous integration, Wafer Level Fan Out (WLFO) packaging has recently seen a tremendous growth in a broad array of applications, from telecommunications and automotive, to optical and environmental sensing, while addressing the challenges of the next big wave of the Internet of Things (IoT). In this context, WLFO is continuously being challenged to include new families of MEMS/NEMS/MOEMS sensors, low thermal budget devices and biochips with microfluidics for biomedical applications. Recent developments in WLFO technology by NANIUM [1] demonstrated the implementation of a keep-out-zone (KOZ) mechanism intended to 1st) protect sensitive sensor areas during the backend processing of WLFO wafers and 2nd) create open zones on the Re-Distribution Layers (RDL). This way, the KOZ mechanism provides a physical, direct path from the embedded device to the environment. This is a necessary feature for environment sensing (e.g., pressure) or to create optical paths free of dielectric and protected from the harsh chemistry steps of the WLFO process. This paper describes new developments on KOZ, implemented with SU-8 photoresist as a WLFO dielectric, whose application is a novelty in the WLFO platform. The use of SU-8 and the KOZ with it, addresses some gaps of the current WLFO technology towards the integration of chips with bio-sensitive areas and sensors with low thermal budget. Due to its well-known bio-compatibility and inert behavior, SU-8 can be used as a neutral dielectric to be in direct contact to target fluids (e.g., sera, blood). Also, due to its low curing temperature, SU-8 allows a very low temperature WLFO process and thus the embedding of temperature-limited devices that have been outside the WLFO realm, for example, magneto-resistive or magnetic-spin sensor chips, which degrades its performance above 160°C. More interestingly, SU-8 exhibits a particular non-conformal behavior, which creates very smooth surfaces even over the mildly rough mold compound area of a fan-out package. Adding to this, SU-8 is readily available in the market in a wide range of thicknesses, spanning from 0.5 μm to >100 μm, and further allowing multiple spin coatings to build thick layers. Thus, SU-8 can provide smooth and deep enough channels for microfluidic flow over the chip sensing areas and, at the same time, provide the necessary layer thickness discrimination for the KOZ mechanism. Combining these features, the SU-8 layers in WLFO can play the triple role of 1) RDL dielectric insulation, 2) KOZ mechanism and 3) embedded microfluidic channels as part of the RDL. In summary, besides the unprecedented use of SU-8 in WLFO packaging, KOZ implementation on SU-8 provides a true, attainable bridge between WLFO and integrated microfluidic applications, for biosensing and biomedical applications in general. Outlooking the potentialities of such a merge, a Fan-Out package can embed several chips interconnected by RDL lines, as it currently allows, and also connected by microfluidic channel for multi-point, multi-function biosensing, constituting a true Lab-on-Package, cost-effective solution. Instead of building all sensing areas and microfluidic channels over a large silicon (Si) chip, this solution builds the feed-in, feed-out areas of the microfluidic channel over the inexpensive fan-out area, minimizing the sensing chip area, with the consequent front-end cost reduction.

Fabrication of high-transmission microporous membranes by proton beam writing-based molding technique

Porous membranes are widely used as filters in a broad range of micro and nanofluidic applications, e.g. organelle sorters, permeable cell growth substrates, and plasma filtration. Conventional silicon fabrication approaches are not suitable for microporous membranes due to the low mechanical stability of thin film substrates. Other techniques like ion track etching are limited to the production of randomly distributed and randomly orientated pores with non-uniform pore sizes.In this project, we developed a procedure for fabricating high-transmission microporous membranes by proton beam writing (PBW) with a combination of spin-casting and soft lithography. In this approach, focused 2MeV protons were used to lithographically write patterns consisting of hexagonal arrays of high-density pillars of few µm size in a SU-8 layer coated on a silicon wafer. After development, the pillars were conformably coated with a thin film of poly-para-xylylene (Parylene)-C release agent and spin-coated with polydimethylsiloxane (PDMS). To facilitate demolding, a special technique based on the use of a laser-cut sealing tape ring was developed. This method facilitated the successful delamination of 20-μm thick PDMS membrane with high-density micropores from the mold without rupture or damage.

Carbon-Electrode Dielectrophoresis for Concentrating Trypanosoma Brucei

Trypanosoma brucei (T. brucei) is a parasite that causes African Trypanosomiasis, or African sleeping sickness. This disease is spread through bite from a Tsetse fly or congenitally. Current tests used to properly diagnose the disease, such as cerebrospinal fluid testing (1) and card agglutination tests (2), are unaffordable in developing countries. For example, impoverished areas in Africa suffering from the disease do not have proper resources to house and maintain equipment needed for these tests. Hence, the goal of this research is to find a more cost-efficient way to identify the disease in rural settings. Dielectrophoresis (DEP) is a separation technique that utilizes the characteristic response of different microorganisms to a non-uniform electric field to segregate a specimen of interest from its background (3). In particular, positive DEP enables the concentration of the targeted organism in regions of high electric field gradient. Here, we postulate the use of inexpensive carbon electrodes to implement positive DEP of T. brucei. We aim at attracting and concentrating T. brucei parasites around the electrodes to facilitate and ease Trypanosomiasis detection by untrained personnel, thus reducing costs and enabling faster treatment of African Trypanosomiasis. The experimental setup used here included a microfluidics chip featuring multiple rows of intercalated carbon microelectrodes as detailed elsewhere (4). The experimental sample was prepared by re-suspending T. brucei parasites in a sugar solution with a conductivity of 500 μS/cm. The sample was flowed through the polarized electrode array at a flow rate of 0.2 μL/min for 2 minutes. After this time the field was turned off and the parasites that may had been trapped were released for elution. The electrode array was polarized with a sinusoidal signal at different frequencies, ranging from 100 kHz to 10 MHz at 100 kHz intervals, but at an optimized constant amplitude of 2 Vpp. The array was monitored to characterize the movement of the parasite around the electrodes and sequential images were taken at the end of the array to characterize the parasite elution pattern once the the field was turned off. These images were analyzed for each experiment to assess the difference in parasite attachment before and after turning the field off. Preliminary results indicate that the use of a voltage value higher than 2 Vpp leads to parasite death. At such polarizing voltage, attraction of the parasite around the electrodes was observed in the frequency range 500-900 kHz. However, trapping is predominantly around the electrically-insulating SU-8 structure around the electrodes. The attraction to this part of the chip, rather than to the carbon electrodes where it was expected, indicates a possible affinity to topography but most likely the possibility of a distorted electric field that surrounds the insulator and generates a high field gradient. Such distortion is the primary hypothesis for the attraction witnessed because we do not see this behavior at other frequencies outside the range 500-900 kHz. While some trapping was observed at experiments run at 10 Vpp, many parasites died when exposed to a high voltage. Shear stress was also examined as a possible cause of parasite death and the flow rate used lead to continued parasite viability. Ongoing work includes investigating the attraction of the parasites to the SU-8 layer, and whether this is caused by the non-uniform electric field around the curvature of the insulator layer. To this end, we are implementing numerical modeling of the electric field in the device to further identify the magnitude of the gradient that causes trapping. Once this is identified, the effect of sample conductivity on the trapping will be addressed to further understand the DEP behavior of T. brucei and facilitate its detection. References Chappuis, F., Loutan, L., Simarro, P., Lejon, V., & Büscher, P. (2005). Options for field diagnosis of human African trypanosomiasis. Clinical Microbiology Reviews, 18(1), 133-146. Chappuis, F., Stivanello, E., Adams, K., Kidane, S., Pittet, A., & Bovier, P. A. (2004). Card agglutination test for trypanosomiasis (CATT) end-dilution titer and cerebrospinal fluid cell count as predictors of human African Trypanosomiasis (Trypanosoma brucei gambiense) among serologically suspected individuals in southern Sudan. The American Journal of Tropical Medicine and Hygiene, 71(3), 313-317. Menachery, M., Kremer, C., Wong, P., Carlsson, A., Neale, S., Barrett M., Cooper, J. (2012). Counterflow Dielectrophoresis for Trypanosome Enrichment and Detection in Blood. Scientific Reports, 775(2), 1-5. Martinez-Duarte, R., Renaud, P., Madou, M. (2011). A Novel Approach to Dielectrophoresis Using Carbon Electrodes. Electrophoresis, 32(17), 2385-2392. Figure 1

Design, fabrication, and characterization of bridge-type micro-hotplates with an SU-8 supporting layer for a smart gas sensing system

In this study, bridge-type micro-hotplates (MHP) with an SU-8 supporting layer were proposed for smart gas sensor applications. The proposed MHP consisted of a heating membrane with an area of 140 µm × 140 µm, and a 33 µm-thick SU-8 layer deposited on its bridges. Finite element method based simulation confirmed that the proposed MHP displayed good thermal isolation properties. The proposed MHP was successfully fabricated, and the properties of the MHP were characterized. Current–voltage characteristics revealed that the MHP temperature can reach 550 °C at 5 V. The temperature of the MHP was calculated from changes in the resistance of the heater. Power consumption of the MHP approximately corresponded to 13.9 mW for heating to 300 °C. This was comparable to the power consumption reported in the previous studies. Furthermore, a stable operation under a constant voltage was observed for 100 min. The properties of the MHP indicated that it could potentially be utilized for applications related to integrated gas sensors.

Microfabricated electrochemical gas sensor

The fabrication and working principle of an electrochemical gas sensor for direct gas phase detection of organic molecules is presented. The sensor is composed of two Pt electrodes where redox reactions can occur, and an ion exchange membrane to conduct H+ from an electrode to the other. As proton exchange membrane they used a spin-coated Nafion® layer. An underlying SU-8 layer assures a double role of adhesion of the Nafion to the substrate and water reservoir to limit dehydration of the Nafion. The sensor is characterised for its electrochemical properties and tested for the detection of CO as model pollutant. The detection tests are performed in the form of cyclic voltammetry, and show that the sensor can detect the gas at the applied voltage corresponding to the oxidation potential of CO into CO2.

Deep proton writing of high aspect ratio SU-8 micro-pillars on glass

Deep proton writing (DPW) is a fabrication technology developed for the rapid prototyping of polymer micro-structures. We use SU-8, a negative resist, spincoated in a layer up to 720μm-thick in a single step on borosilicate glass, for irradiation with a collimated 12MeV energy proton beam. Micro-pillars with a slightly conical profile are irradiated in the SU-8 layer. We determine the optimal proton fluence to be 1.02×104μm−2, with which we are able to repeatably achieve micro-pillars with a top-diameter of 138±1μm and a bottom-diameter of 151±3μm. The smallest fabricated pillars have a top-diameter of 57±5μm. We achieved a root-mean-square sidewall surface roughness between 19nm and 35nm for the fabricated micro-pillars, measured over an area of 5×63.7μm. We briefly discuss initial testing of two potential applications of the fabricated micro-pillars. Using ∼100μm-diameter pillars as waveguides for gigascale integration optical interconnect applications, has shown a 4.7dB improvement in optical multimode fiber-to-fiber coupling as compared to the case where an air–gap is present between the fibers at the telecom wavelength of 1550nm. The ∼140μm-diameter pillars were used for mold fabrication with silicone casting. The resulting mold can be used for hydrogel casting, to obtain hydrogel replicas mimicking human tissue for in vitro bio-chemical applications.

Plasma functionalization procedure for antibody immobilization for SU-8 based sensor

In this paper, we report the study on a new protocol for the immobilization process of antigen/antibody assay on SU-8 layers by oxygen plasma treatment. Plasma treatments, at different plasma powers and for different duration times, are performed and their effects on immobilization efficiency are studied. The chemical properties and the surface morphology of SU-8 before and after the functionalization and immobilization of (IgG) are then verified by Raman spectroscopy and atomic force microscopy (AFM). An increase of the surface roughness of SU-8 layers is observed after the oxygen plasma treatment and an intensity variation of functional groups is also evidenced. To demonstrate the validity of the process the distribution of IgG immobilized on SU-8 surfaces is detected by fluorescence microscopy measurement after incubation with fluorescein isothiocyanate (FITC)-tagged anti-human IgG. An increase of the amount of the adsorbed protein of about 20% and a good repeatability on antigen/antibody distribution on the surface are detected for IgG on plasma treated substrates. Finally, label free measurements are performed by SU-8 optical ring resonators reaching detection limits of 0.86ngcm−2. The proposed approach offers a smart protocol for IgG immobilization on SU-8 substrate that can be easily extended to different antigen/antibody assay and polymeric materials for the realization of high performance immunosensors.

UV-LIGA microfabrication process for sub-terahertz waveguides utilizing multiple layered SU-8 photoresist

A microfabrication process based on UV LIGA (German acronym of lithography, electroplating and molding) is proposed for the fabrication of relatively high aspect ratio sub-terahertz (100–1000 GHz) metal waveguides, to be used as a slow wave structure in sub-THz vacuum electron devices. The high accuracy and tight tolerances required to properly support frequencies in the sub-THz range can be only achieved by a stable process with full parameter control. The proposed process, based on SU-8 photoresist, has been developed to satisfy high planar surface requirements for metal sub-THz waveguides. It will be demonstrated that, for a given thickness, it is more effective to stack a number of layers of SU-8 with lower thickness rather than using a single thick layer obtained at lower spin rate. The multiple layer approach provides the planarity and the surface quality required for electroforming of ground planes or assembly surfaces and for assuring low ohmic losses of waveguides. A systematic procedure is provided to calculate soft and post-bake times to produce high homogeneity SU-8 multiple layer coating as a mold for very high quality metal waveguides. A double corrugated waveguide designed for 0.3 THz operating frequency, to be used in vacuum electronic devices, was fabricated as test structure.The proposed process based on UV LIGA will enable low cost production of high accuracy sub-THz 3D waveguides. This is fundamental for producing a new generation of affordable sub-THz vacuum electron devices, to fill the technological gap that still prevents a wide diffusion of numerous applications based on THz radiation.

Stress reduction method in fabrication of a multi-scale inertial switch with ultra-high aspect ratio

Micro inertial switches are widely used in airbags, accessories and military weapons. SU-8 photoresist and electroforming based UV-LIGA process is an effective way to fabricate inertial switches. However, during the fabrication, the high stress in the SU-8 layers often causes them to detach from the substrate, which results in fabrication failure. To solve the detachment problem of SU-8 layers, a novel method was presented in this paper to release the internal stress in SU-8 layers by decreasing the area of electroforming layers. ANSYS simulation study was performed to verify the stress reduction effect of this method. The simulation and experiment results showed that stress in SU-8 layers can be reduced effectively. By using this method, a multi-scale inertial switch with ultra-high aspect ratio 17:1 was fabricated successfully. The overall size of the inertial switch is 14 × 11 × 0.6 mm. The inertial switch is fabricated by multi-layer UV-LIGA process. Fabrication error of the process is estimated in this paper. The stress reduction method and error estimate method presented in this paper can provide help to the fabrication of micro metal devices with large area and high aspect ratio.

Tracking Electrochemical Processes at the Nanoscale: Operando Transmission Electron Microscopy of Lithium Dendrite Formation

While lithium metal possesses one of the highest specific energy storage densities for secondary batteries at 3.86 A-hr g-1, its use is problematic. Li metal anodes are known to form dendrites during charging cycles and cause device failure. If the dendrite extends to the cathode it shorts and heats up the battery to the flashpoint of the solvent and creates a pressure build up in the casing. The device becomes ripened for fire and explosion. Here we focus on the direct imaging of first the formation of the solid-electrolyte interphase (SEI) on a glassy carbon electrode using in situ electrochemical scanning transmission electron microscopy (ec-STEM) [1]. After SEI formation we follow the early stages of Li dendrite nucleation and growth and speculate on the role that the SEI has upon Li electrodeposition and dendrite growth. We expand the used of Z-contrast scanning TEM imaging to estimate the density of the SEI and correlate the amount of lithium deposited onto the electrode in view with the current measured during the cyclic voltammogram. We are able to record high angle annular dark field (HAADF) STEM images of SEI formation and Li deposition by utilizing a microfluidic electrochemical cell that is placed within a tip of a TEM holder. We will briefly discuss the small-scale STEM microfluidic electrochemical liquid cell platform which is formed by compressing two silicon microchip devices together. Each microchip supports a 50 nm thick electron-transparent silicon nitride membrane with a rectangular viewing area of 50 x 200 μm. Three platinum electrodes is patterned one microchip's surface with the central electrode having glassy carbon deposited on it to be used as a working electrode. The lower microchip has a layer of SU-8 that acts as a spacer and flow channel (400 μm wide, 800 μm in the vacuum TEM environment) for transport of electrolyte between the two microchips when inserted into the TEM holder. Battery-grade 1.2 M LiPF6EC:DMC electrolyte was delivered to the microfluidic electrochemical cell by a syringe pump. In this work, HAADF STEM imaging was used to facilitate Z-contrast conditions, which allows for quantitative image analysis, to estimate thickness and density of the SEI layer and Li deposits. It also allows for providing definitive proof of Li electrodeposition though contrast analysis. The electrochemical window of the electrolyte is generally taken as 1.2 to 4 V vs Li/Li+. We show that during a thin film developed on the glassy carbon's edge during potential cycling more negative than 1.5 V due to the reduction of the ethylene carbonate. EC reduces to carbonate, ethylene gas, and intermediate radical species that can polymerize. The dynamics of the SEI in response to the change in electric field is directly imaged; the SEI swells and becomes denser with each potential cycle. We also show how Li dendrites seem to grow from defects in the SEI that form about the deposits which suggests that there is a competition between EC reduction to Li2CO3(or some other passivating species) and Li electrodeposition. Research supported by the Fluid Interface Reactions Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the Department of Energy’s Office of Basic Energy Sciences Division and by the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. [1] Sacci, R. L., Black, J. M., Balke, N., Dudney, N. J., More, K. L., & Unocic, R. R. (2015). Nanoscale imaging of fundamental li battery chemistry: solid-electrolyte interphase formation and preferential growth of lithium metal nanoclusters. Nano Letters, 15(3), 2011–2018.

Fabrication of Vertical Molecular Junction Devices with Conductive Polymer Contacts Using a Peeling Method.

PSS), as a contact layer between the self-assembled monolayer and the Au top electrode. In this method, a thermally curable photoresist of SU-8 is used to define the areas where the top electrodes are formed. The hydrophobicity and low surface energy of the cured SU-8 facilitates selective deposition of PSS onto the defined top electrode areas of the device through solution dewetting, and also enables the physical peeling of the Au top electrodes deposited on the SU-8 layers using an adhesion tape. Through this approach, vertical molecular junctions with patterned Au top electrodes can be fabricated without employing the shadow mask evaporation process used in the conventional approach. We processed vertical molecular tunneling devices based on self-assembled alkanedithiol monolayers with an active area of 100 µm2.

Pyrolytic 3D Carbon Microelectrodes for Electrochemistry

This work presents the fabrication and characterization of multi-layered three-dimensional (3D) pyrolysed carbon microelectrodes for electrochemical applications. For this purpose, an optimized UV photolithography and pyrolysis process with the negative tone photoresist SU-8 has been developed. The fabricated three electrode electrochemical cell is characterized with cyclic voltammetry (CV) using the standard potassium ferri-ferrocyanide redox probe. Carbon materials have several attractive characteristics as microelectrodes for electrochemical applications, such as wide potential window, good electrochemical activity, chemical stability, and ease in surface functionalization [1]. The most common carbon microfabrication techniques (i.e. screen printing) produce two-dimensional (2D) electrodes, which limit the detection sensitivity. Hence several 3D microfabrication techniques have been explored in recent years amongst which the carbon MEMS (C-MEMS) technique is the most promising one. C-MEMS is a simple and cost-effective method for carbon electrode fabrication, where a patterned polymer template treated at high temperature (~900°C) in inert atmosphere (N2 or Ar) is transformed into pyrolysed carbon [2]. This process enables fabrication of 2D and 3D electrodes with possibility for tailoring ad-hoc designs and unique sensitivities for specific applications. Due to this, pyrolysed carbon is becoming increasingly attractive for numerous applications, such as novel sensors and scaffolds for cell analysis [3]. However fabrication of a conducting 3D microstructure with feature sizes in the micron-range still remains a challenge. In this work an optimized UV photolithography and pyrolysis process for SU-8 based on highly controlled exposure dose and modified baking time is presented to obtain multi-layered 3D carbon microelectrodes for electrochemistry (figure 1.A). SU-8 2005 (5.6µm) is spin coated on a Si/SiO2 wafer, soft baked (SB) at 50ᵒC for 30min followed by UV exposure (E1 – 210mJ cm-2) and post exposure bake (PEB) at 50ᵒC for 1h (figure 1.A.a). A second thick layer of SU-8 2075 (76µm) is spin coated, SB at 50ᵒC for 6h and UV exposed (E2 – 149mJ cm-2) (figure 1.A.b). A second partial exposure E3 – 28mJ cm-2 is performed to obtain a suspended layer followed by a PEB at 50ᵒC for 8h (figure 1.A.c). The partial exposure dose at wavelength 365nm and the low temperature (50ᵒC) baking steps plays a critical role in fabricating suspended layer. Non cross-linked SU-8 is developed in propylene glycol monomethyl ether acetate (PGMEA) for 30min (figure 1.A.d). The obtained SU-8 polymer templates are then pyrolysed at 900ᵒC for 1h in an N2 environment to obtain suspended 3D pyrolysed carbon microelectrodes (figure.1.A.e). By sequentially repeating the steps shown in figure 1.A. b, c and d followed by a final development step, a multi-layered polymer template can be obtained which can be pyrolysed to produce 3D carbon microelectrodes (figure 1.A.f). The height of the singe carbon layer (figure 1.A.e) is 21.10µm which includes a suspended layer of 2.25µm. The diameter of circles in the suspended layer is 11.50µm. The small carbon microstructures are shown in figure 1.B with structures of 4µm (mesh) and 18µm (mesh boundaries). Depending on the desired application, different 3D carbon microelectrodes can be fabricated (figure 1.B). Figure 1.C shows the electrochemical characterization of a three electrodes system, comparing planar and 3D carbon working electrodes. The cyclic voltammograms performed in 10mM ferri-ferrocyanide show higher peak current (2 folds higher) for the 3D microelectrodes compared to the 2D ones (figure 1.C). The 3D microelectrodes potentially increase the overall sensitivity in amperometric monitoring of cell response due to the increase surface area and enhanced interaction with cells [3]. Reference: [1] R. L. McCreery, “Advanced carbon electrode materials for molecular electrochemistry,” Chem. Rev., vol. 108, no. 7, pp. 2646–2687, 2008. [2] R. Martinez-Duarte, “SU-8 Photolithography as a Toolbox for Carbon MEMS,” Micromachines, vol. 5, no. 3, pp. 766–782, 2014. [3] L. Amato, A. Heiskanen, C. Caviglia, F. Shah, K. Zór, M. Skolimowski, M. Madou, L. Gammelgaard, R. Hansen, E. G. Seiz, M. Ramos, T. R. Moreno, A. Martínez-Serrano, S. S. Keller, and J. Emnéus, “Pyrolysed 3D-Carbon Scaffolds Induce Spontaneous Differentiation of Human Neural Stem Cells and Facilitate Real-Time Dopamine Detection,” Adv. Funct. Mater., vol. 24, no. 44, pp. 7042–7052, 2014. Figure 1